A tensioning device, such as a hydraulic tensioner, is used to impart and maintain a certain degree of tension in the chain or belt that drives, for example, an engine camshaft. A hydraulic tensioner typically comprises a housing having a bore, and a piston having a rear end which is inserted into the bore of the housing. The piston is biased in a protruding or outward direction by a spring. A fluid chamber is formed by the inner wall surface of the bore and the rear end face of the piston. A fluid pressure source provides pressurized fluid through a passage in the housing to the fluid chamber.
When the tension of a chain or belt increases during engine operation, the piston receives a force in the retraction direction. For example, when the fluid chamber has an insufficient oil level or pressure, such as immediately after engine start-up or at low engine rpm, the piston retracts and the proper tension cannot be maintained for the chain or belt.
Hydraulic tensioners having a ratchet mechanism have been proposed to reduce or limit the retraction of the piston. For example, a hydraulic tensioner having a pin member attached to the rear end of the piston, wherein the pin member has an inside ratchet that engages with an engagement aperture is disclosed in Japanese Laid-Open Patent Heisei 6-42598. Because the force acting on the piston in the retraction direction is absorbed by the pin member and the engagement aperture, both the pin member and aperture tend to weaken after repeated extension and retraction of the piston. Moreover, in such a tensioner, increasing the thickness of the ratchet and pin member to increase durability would require enlargement of the housing.
The present invention is directed to the above-described problems in conventional hydraulic tensioners, and offers a hydraulic tensioner which further increases durability and conserves housing space.